Same evidence suggesting Britney Spears would not sell out a stadium 10 years after her prime with casuals, Paulie Shore movie wouldn't be a box office hit 10 years after his prime with casuals, and Phil Donahue wouldn't be an internet politics star 10 years after his prime with casuals.
The amount of names on a list called "10 years after casuals are into you, they are still really into you" is basically a blank sheet of paper.
How is Oasis' tour going? Didn't they break up in 2009 and peak with casuals well before then? Are the 100k+ stadium shows just filled with hardcores? Coldplay apparently had the #2 tour in 2024, behind only Taylor Swift...but wouldn't their peak be something like 15 years ago? Chalk that up to hardcores again, I guess? Look at the other top tours of recent years and see how your theory of '10 years after their prime with casuals' holds up.
Movies...Deadpool vs. Wolverine was #2 last year...but that was six years after the last Deadpool film and wasn't Wolverine/X-Men's 'prime with casuals' a good 15 or so years ago too? Another fluke, I guess. Let's really get into the weeds...the 'Bad Boys' films of 2020 and 2024...quite the resurgence after the 1995 and 2003 films...and aren't Will Smith and Martin Lawrence well removed from their 'prime with casuals'? I think Conor was just a wee lad during Martin's hey day (his TV show is still one of the most underrated comedies of its time, IMHO).
I think your "blank sheet of paper" would fill up pretty fast if we keep going and look further at the film, music, and TV/streaming industries.
Yes, many people certainly do flame out or simply fall victim to changing tastes and/or their declining talent...but many endure or even make wild comebacks. There is no 'rule' and each should be looked at on an individual basis. Consider too that with something like music or film, you can sample the recent product with a single or a trailer, to decide if you want to buy the album, attend a concert, etc. Not many people chose to see a film or buy an album where they are confident they will not be entertained. There is also enough volume of quality alternatives to which consumers can shift their interest quickly.
The MMA and combat sports space is different...and Conor is different. There is not a high volume of 'big event' fighters like Conor, Brock, Ronda, etc. that have their level of appeal. He is also unique amongst MMA fighters...the Taylor Swift of MMA...in that he has been head and shoulders above the rest in terms of drawing power, earnings, notoriety, etc. Unlike declining music or film artists, there is also better reason to believe that many people WOULD tune in with the hopes of seeing him fail.
Don't kid yourself...the interest amongst the wider audience has not completely evaporated. It might be reduced, but Conor will still be a top (if not *the* top) draw for the UFC if he chooses to fight. As I've said before, the real question is not whether he will draw, it is whether he will actually make the walk again.